1. Technical Field
This invention is directed to polymeric compositions having improved resistance to flame spread and smoke evolution as well as improved physical properties. Such compositions are useful in forming primary insulation and protective jacketing for electrical conductors such as wire and cable, and are also suitable for buffering optical fiber. More particularly, this invention relates to flame and smoke resistant vinyl halide polymers having reduced rates of heat and smoke release and improved low temperature impact properties and dynamic thermal stabilities.
2. State of the Art
Because of today's need for the instantaneous transmission of data and information, reliance upon computer, word processing, electronic data transmission, sensor and telecommunications equipment is increasing at a phenomenal pace. The advent of technology is making it possible to interface and network this equipment, leading to the development of integrated communications systems. Such systems are increasingly installed in confined areas such as in high rise buildings, watercraft, aircraft, trains, drilling platforms and in mines. In order to link these electronic components, it is necessary to install thousands of feet of wire and cable throughout these structures and areas. Typically, it is a convenient practice to install the wire and cable through air handling plenums and wire and cable raceways. Because plenums and raceways are continuous throughout these installations, it is essential for safety that the insulation and jacketing materials have low flame spreading and smoke evolving properties as well as diverse physical properties for performance under a variety of hostile environmental conditions.
Polyvinyl chloride (PVC) has traditionally been the material of choice for wire and cable insulation because of its inherent flame resistant properties. PVC is well-known to be one of the least ignitable of the polymeric materials and once ignited it is one of the least flammable. Other desirable attributes include mechanical toughness, resistance to chemical corrosion, and good dielectric properties. Additionally, PVC is relatively low in cost. A drawback, however, is that PVC is a rigid thermoplastic that lacks flexibility. Upon exposure to temperature extremes, PVC loses its resistance to high heat distortion and low temperature brittleness. Another disadvantage is that PVC is highly viscous at processing temperatures without the use of plasticizing additives, and is therefore difficult to compound and process. Accordingly, plasticizers are added to PVC during processing to improve the processing characteristics and the flexibility of the end product. However, the use of plasticizers in wire and cable insulation applications is limited in that, plasticizers reduce the flame resistance, increase smoke evolution and impair the dielectric properties of the insulating material.
In order to overcome some of these disadvantages, it is known to blend PVC with chlorinated polyolefins and more particularly chlorinated polyethylene (CPE). For example, U.S. Pat. No. 3,845,166 discloses a wire and cable insulation composition comprising PVC, a chlorinated polyolefin, polyethylene and a crosslinking agent. This thermosetting composition may additionally contain various additives such as pigments, antioxidants, stabilizers and the like.
U.S. Pat. No. 4,129,535 discloses fire retardant PVC film compositions. The films comprise a blend of PVC, chlorinated polyethylene, a phosphate ester plasticizer, a magnesium hydroxide filler as well as zinc borate and antimony trioxide fire retardants.
In U.S. Pat. No. 4,280,940 there is disclosed a thermoplastic composition comprising PVC and a mixture of two differently chlorinated polyethylenes. The composition may additionally contain additives such as heat and light stabilizers, UV absorbers, lubricants, plasticizers, pigments and antistatic agents.
U.S. Pat. No. 4,556,694 discloses a method for improving the low temperature properties of PVC by adding a chlorosulfonated polyethylene and a chlorinated polyethylene. This patent teaches a synergistic effect between the CPE and chlorosulfonated polyethylene is necessary to achieve improved low temperature brittleness properties.
However, these compositions are lacking in that a combination of superior flame and smoke suppression and good low temperature performance, e.g., brittleness temperature, are not achieved. In addition, dynamic thermal stability (DTS) at medium and high shear rates is poor for the aforementioned compositions.
Another method being utilized to provide fire resistance to insulated wire and cable involves surrounding the insulated primary conductors with fire retardant jacketing materials. U.S. Pat. No. 4,401,845 discloses a cable comprised of a bundle of conductors which are insulated with a coating of poly (vinylidene fluoride)(PVDF), a sheath of poly (tetrafluoroethylene) (PTFE) impregnated glass wrap surrounding the bundle of insulated conductors and an outer protective jacket of PVDF. Although fluoropolymers have good resistance to flame and smoke spread, the inherently high dielectric constant of PVDF renders it unsuitable for many applications, such as, for example, in primary insulation for wire used in telecommunications cable. Moreover, fluoropolymers are relatively expensive.
Some of the disadvantages of prior fluoropolymer cable insulation construction have been overcome with the advent of PVC/PTFE/PVDF insulated cable construction. In U.S. Pat. No. 4,605,818 there is disclosed a cable construction comprising at least one conductor which is insulated with PVC coating, a sheath of woven glass impregnated with PTFE, and an outer protective jacket of PVDF. Although this cable construction provides good dielectric, flame and smoke resistant properties, there still is a need for wire and cable insulation with improved flame and smoke resistance.
As more stringent fire and safety standards are enacted, wire and cable insulation will require enormously improved fire and smoke performance while maintaining superior physical properties. There is therefore a need for a low cost wire and cable insulation composition having improved flame spread and smoke evolution characteristics while simultaneously having superior physical properties.